Remove dependency maximum version constraints

This release is compatible with PyYAML v4.2 betas, so users can use a version of PyYAML that does not have the CVE-2017-18342 vulnerability.

Furthermore, this release removes the maximums from dependency version constraints, in order to reduce the maintenance burden. It also has a few bugfixes, increases test coverage, and fixes a warning condition with Python 3.

• Allow space in mimetype before parameters (thanks to miki725)
  #206
• Open up dependency requirements (thanks to blueyed, javabrett)
  #207
  #211
• Increased test coverage with additional vectors, rearranged
  #214
  #216
• Fixed warning about importing from collections on Python 3 (thanks to sloria)
  #215